A method for preparing a gallium arsenide semiconductor photoconductive switch electrode

Abstract

The present invention relates to a method for preparing a gallium arsenide semiconductor photoconductive switch electrode, comprising the following steps: (1) using a dicing machine to slice a wafer of a required size as a substrate; (2) cleaning the surface of gallium arsenide; (3) closely attaching a stainless steel metal mask frame to one surface of gallium arsenide; (4) coating as one electrode; (5) closely attaching a stainless steel metal mask frame to the other surface opposite to gallium arsenide; (6) coating as another electrode; (7) placing the switch chip with metal electrodes on both sides in a rapid annealing furnace for stress relief annealing and alloy treatment. The present invention prepares metal electrodes by using photolithography and etching technology, reduces electrode edge shadows and burrs, alleviates the electric field enhancement effect, improves the working withstand voltage performance of gallium arsenide photoconductive switch without changing raw materials and other process conditions, improves the bonding force between electrode and gallium arsenide through a rapid annealing process, eliminates heterogeneous interface defects, and improves the performance of gallium arsenide photoconductive switch.

Description

Technical Field

[0001] The invention relates to the technical field of semiconductor photoconductive switches, and in particular to a method for preparing two surface electrodes of a gallium arsenide photoconductive switch. Background Art

[0002] Photoconductive semiconductor switches (PCSS) are a new type of semiconductor optoelectronic device developed on the basis of microelectronics technology, laser technology and microwave technology. They have been widely used to generate high-power ultrafast electrical pulses. PCSS technology is a revolutionary breakthrough in the history of high-power ultra-wideband pulse generation and application. Subsequently, the materials used for PCSS were further explored, and diamond, silicon carbide and other PCSS with excellent performance were developed. With the research on photoconductive switches, it has become a realistic possibility for their miniaturization, practicality and integration. In recent years, they have been widely used in high-current ignition device drivers, anti-interference radar microwave bombs, laser nuclear fusion systems, electronic countermeasures and high-power pulse devices, attracting widespread attention from all walks of life.

[0003] For photoconductive switches, response speed, switching efficiency, and transmission power are extremely important factors. The materials for photoconductive switches need to have a shorter carrier lifetime and a higher carrier mobility. The bandgap width of gallium arsenide is 1.42ev, which is a direct bandgap semiconductor material, and the corresponding intrinsic absorption wavelength is 876nm. Due to the impurities doped inside the GaAs material, body defects are formed, and deep energy levels that affect the conductivity of GaAs are generated, so that the GaAs photoconductive switch can produce non-intrinsic absorption of 905nm lasers. The dark state resistance of the GaAs photoconductive switch is as high as several GΩ, and the on-resistance is as low as less than 1Ω, with excellent electrical properties. GaAs has a higher bandgap width and the highest mobility, a higher dark resistivity, and a short carrier lifetime. The rise time of the output electrical pulse mainly depends on the width of the excitation light pulse, and the fall time mainly depends on the carrier lifetime. Experimental studies have shown that for common excitation light pulse wavelengths, photoconductive switches made of GaAs materials have higher photoelectric conversion efficiency. In the absence of light, the resistivity is high, the current is small, and the switch is in a blocked state. When there is light, a large number of electron-hole pairs are generated in a very short time, the resistivity decreases, and the photoconductive switch is in the on state.

[0004] At present, the manufacturing structures of photoconductive switches include surface structures and body structures, namely coplanar switches and skew switches. Coplanar switches refer to photoconductive switches in which the two electrodes are processed on the same surface of the semiconductor material. They have a simple structure, but poor voltage resistance. Under the action of a strong bias electric field, the rectangular corners are prone to breakdown of the air. Skew photoconductive switches refer to photoconductive switches in which the two electrodes are processed on different surfaces of the semiconductor material. This can increase the voltage resistance exponentially, making the photoconductive switch miniaturized and integrated. Gallium arsenide body structure photoconductive switches have more superior performance, such as smaller size, stronger voltage resistance, and more stable performance. They are an excellent choice for manufacturing photoconductive switches and are of great significance for improving the working performance and voltage resistance of photoconductive switches. Patent CN106816495A introduces that the electrodes of gallium arsenide switches use photoresist as a mask, and the switch electrodes are prepared by plating a metal film and then removing the glue, that is, the electrodes are prepared by lift-off processing. The thickness of the electrodes prepared by this method is limited, and it is difficult to form a high-voltage, high-power gallium arsenide switch under the same conditions. Summary of the invention

[0005] In view of the problems in the related technology, the present invention proposes a method for preparing a gallium arsenide semiconductor photoconductive switch electrode to overcome the above-mentioned technical problems existing in the existing related technology. The purpose of the present invention is to process a thicker metal electrode through a metal mask with a smooth edge prepared by photolithography and etching technology, reduce the shadow and burr of the electrode edge, alleviate the electric field enhancement effect, increase the thickness of the metal electrode, and improve the working voltage resistance performance of the gallium arsenide photoconductive switch without changing the substrate and other process conditions; improve the bonding strength between the electrode and gallium arsenide through a rapid annealing process, eliminate heterogeneous interface defects, and thus improve the performance of the gallium arsenide photoconductive switch.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a gallium arsenide semiconductor photoconductive switch electrode, comprising the following steps:

[0008] (1) Using a dicing machine to cut a wafer of the required size as a substrate;

[0009] (2) Cleaning the surface of gallium arsenide;

[0010] (3) A stainless steel metal mask frame is attached to a surface of the gallium arsenide; the stainless steel metal mask frame is processed into a pattern required for the switch electrode by semiconductor photolithography and etching methods;

[0011] (4) The coating is used as an electrode; the coating technology adopts vacuum electron beam evaporation coating technology or magnetron sputtering technology to sequentially coat Ge, Ni and Au films; the coating technology adopts vacuum electron beam evaporation coating technology or magnetron sputtering technology;

[0012] (5) A layer of stainless steel metal mask frame is closely attached to the other surface opposite to the gallium arsenide; the stainless steel metal mask frame is processed into the pattern required for the switch electrode by semiconductor photolithography and etching methods;

[0013] (6) coating as another electrode; the coating technology adopts vacuum electron beam evaporation coating technology or magnetron sputtering technology, and Ge, Ni and Au films are coated in sequence; the coating technology adopts vacuum electron beam evaporation coating technology or magnetron sputtering technology;

[0014] (7) The switch chip with metal electrodes on both sides is placed in a rapid annealing furnace for stress relief annealing and alloy treatment.

[0015] Preferably, in step (3) and step (5), the edges of the electrode pattern in the stainless steel metal mask frame are smooth, free of burrs and sharp right angles.

[0016] Preferably, the thicknesses of the Ge film, the Ni film and the Au film are 5-200 nm, 50-4000 nm and 200-2000 nm respectively.

[0017] Preferably, in step (4) and step (6), an Au film is plated, and the thickness of the Au film is 200-2000 nm.

[0018] Preferably, in step (7), the stress relief annealing treatment is performed in a rapid annealing furnace, and the temperature of the stress relief annealing is 250-480°C.

[0019] Preferably, the insulation time is 30s to 900s.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The present invention is a method for preparing a gallium arsenide semiconductor photoconductive switch electrode. The present invention processes a thicker metal electrode by using a stainless steel metal mask frame with smooth edges prepared by photolithography and etching technology to reduce electrode edge shadows and burrs, alleviate the electric field enhancement effect, increase the thickness of the metal electrode, and improve the working voltage resistance performance of the gallium arsenide photoconductive switch without changing the substrate and other process conditions;

[0022] (2) The present invention is a method for preparing a gallium arsenide semiconductor photoconductive switch electrode. The present invention improves the bonding strength between the electrode and gallium arsenide through a rapid annealing process, eliminates heterogeneous interface defects, and thus improves the performance of the gallium arsenide photoconductive switch. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the effect of Au film coating on the surface of a 3 mm electrode of the present invention;

[0024] Figure 2This is a schematic diagram of the effect of Au film coating on the surface of the 3mm electrode and the 2mm electrode of the present invention;

[0025] Figure 3 This is a rendering of the etching effect of the gallium arsenide photoconductive switch of the present invention;

[0026] Figure 4 This is a diagram showing the effect of the first gallium arsenide photoconductive switch of the present invention after annealing;

[0027] Figure 5 This is a diagram showing the effect of the second gallium arsenide photoconductive switch of the present invention after annealing. DETAILED DESCRIPTION

[0028] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] Example

[0030] A method for preparing a gallium arsenide semiconductor photoconductive switch electrode comprises the following steps:

[0031] Surface electrode Au

[0032] Preliminary cleaning → front electrode vacuum coating → photolithography → etching → back electrode vacuum coating → annealing

[0033] 1. Preliminary cleaning

[0034] (1) The importance of cleaning

[0035] Before manufacturing semiconductor devices, the substrate must be cleaned. Semiconductors must be cleaned before oxidation, coating, photolithography and other processes. Cleaning refers to the use of physical or chemical methods to remove surface organic matter or oxides to achieve surface cleanliness requirements.

[0036] With the integration and miniaturization of semiconductor devices, if the cleaning is not good, the performance of the product will also decline. Therefore, higher requirements are put forward for the cleaning of semiconductor surfaces, and even ultra-clean surface requirements are put forward. Cleaning requires small surface roughness, effective removal of surface organic matter or oxides, and no damage to the substrate surface when using physical or chemical methods.

[0037] The reason why such high requirements are placed on cleaning is:

[0038] ① The presence of impurities on the surface will affect the conductivity of the semiconductor, which can easily increase the surface leakage current, cause crystal defects, and affect the performance of the product. Poor cleaning of surface organic matter will affect the subsequent epitaxial growth of the crystal. Crystal defects are easily generated during coating. In addition, the residual organic matter on the surface will evaporate into the film layer as the temperature rises, resulting in poor quality of the coating electrode, affecting subsequent photolithography, etching and other processes, and reducing the device yield;

[0039] ② Impurity contamination can easily change the breakdown voltage, forming leakage current on the surface and causing soft breakdown;

[0040] ③Impurity contamination affects carrier lifetime and device performance.

[0041] During cutting, grinding, polishing and other mechanical processes, the GaAs substrate comes into contact with various greases and adhesives, and there are many organic pollutants on the surface. In addition, the GaAs edge is worn on the surface during the cutting process. In summary, in order to ensure device performance, early cleaning is essential.

[0042] (2) Gallium arsenide surface cleaning process

[0043] This experiment uses the chemical cleaning method, the specific operation is as follows:

[0044] ① Soak the GaAs substrate in 1# solution (NH3.H2O:H2O2:H2O=1:2:8) for 2 minutes;

[0045] ② Rinse in deionized water;

[0046] ③ AR acetone soak for 10 minutes;

[0047] ④Soak in AR ethanol for 10 min;

[0048] ⑤ Rinse under deionized water;

[0049] ⑥ Blow dry with N2;

[0050] ⑦ Place the GaAs substrate in a 100°C oven to dry.

[0051] 2. Vacuum coating

[0052] (1) Principle of vacuum coating

[0053] Vacuum coating is a type of physical vapor deposition method and is the earliest coating technology. It is mainly composed of a vacuum chamber, a mechanical pump, a molecular pump, a vacuum compound meter, a crystal oscillator, and a condensation system. The most commonly used is a turbomolecular pump, which is composed of thousands of blades and can rotate at a speed of up to 21,000 rpm. It is a vacuum pump that transfers momentum to the gas through high-speed rotation to evacuate the gas.

[0054] The mechanical pump often used is a rotary vane mechanical pump, which is used to extract the dry gas in the vacuum chamber.

[0055] The vacuum compound gauge is used to detect the air pressure in the vacuum chamber. The main component of the crystal oscillator is the crystal oscillator, which is used to detect the coating thickness and coating rate, and more accurately reflect the coating conditions in the vacuum chamber.

[0056] Vacuum coating mainly uses the high energy of electron beam to heat and melt metals or metal compounds and non-metallic materials under high vacuum conditions, and then evaporates and deposits them on the surface of the plated part on the work turntable to form a uniform and continuous thin film.

[0057] The film layer produced by vacuum evaporation coating is uniform and dense, the film thickness is controllable, the deposition rate is fast, mass production can be carried out, and a wide variety of evaporated and deposited metals can be produced. However, vacuum coating is not suitable for evaporating high-melting-point metals, and the composition ratio is uncontrollable.

[0058] (2) GaAs front electrode coating process

[0059] like Figure 1-2 As shown, Au film is plated on the surface of 3mm electrode and 2mm electrode respectively. In this experiment, ZZS500-4 / G vacuum coating machine is used to make pure Au metal film. Vacuum coating technology is used, and ion source is added to assist coating, in order to increase the adhesion between the coating material and the substrate. Finally, electron beam evaporation is used to deposit the metal on the substrate to form a metal electrode. The thickness of the pure gold electrode of the ohmic contact of the gallium arsenide switch is 1000-6000nm; the coating of the front electrode and the back electrode of different types of switches is completed twice, and the back electrode is the same as the front electrode, and the same coating method is used to prepare the metal electrode.

[0060] The specific operation process is as follows:

[0061] ① Turn on the main power of the laboratory, then turn on the gas switch of the vacuum coating machine, the power switch and the condenser. Deflate the air, and then clean the vacuum chamber. Turn on the mechanical pump and start the mechanical pump. Open the pre-valve, open the high valve, start to draw a high vacuum, and wait until the vacuum degree reaches 5x10^ -3 Around pa, the coating work begins;

[0062] ② When the cleaning time is up, adjust the crystal oscillator program → turn on the power → start the filament → preset → 6KV → high voltage → add beam current for pre-melting → open the baffle;

[0063] ③After the coating is finished, turn off the precision controller, and turn off the baffle, beam, high voltage, gun filament, and power supply in sequence, and wait for the temperature of the vacuum chamber to drop so that it is convenient to take samples;

[0064] ④ When the temperature in the vacuum chamber cools down to about 35℃, turn off the vacuum compound meter, close the high valve, stop the molecular pump, wait until the speed drops to 0, turn off the molecular pump, close the pre-valve, and then turn off the mechanical pump, open the vent valve, and finally take out the sample, clean the vacuum chamber, and then evacuate the vacuum chamber to keep a low vacuum state when no coating is being performed.

[0065] Table 1-1 Vacuum coating parameters

[0066]

[0067] Table 1-1 shows the parameters of vacuum coating of metal electrode materials.

[0068] 3. Lithography

[0069] (1) Principles of photolithography

[0070] Photolithography is the most important process in semiconductor manufacturing. If photolithography is not successful, subsequent processes will not be able to proceed. It plays a decisive role in the minimum feature size of the chip. Photolithography uses ultraviolet light to transfer the image on the mask to the chip. The chemical properties of the photoresist are changed by ultraviolet light irradiation, which facilitates the subsequent etching process and successfully transfers the electrode pattern.

[0071] In the photolithography process, commonly used chemical solvents are photoresist and developer. There are two types of photoresist: positive photoresist and negative photoresist. Photoresist is photosensitive and denatures when exposed to ultraviolet light, so it can only be effectively removed in the developed part. Positive photoresist has high resolution and the exposed part is dissolved, while negative photoresist is cheap and the unexposed part is dissolved. Whether to use positive photoresist or negative photoresist depends on the electrode pattern, and the electrode patterns of the two are opposite. The developer removes the photoresist to be dissolved to facilitate the completion of the subsequent etching process.

[0072] (2) Lithography operation process

[0073] ① Spreading the photoresist on the positive electrode surface of GaAs: Place the GaAs substrate on the spreader, and drop the positive photoresist on the positive surface of GaAs. Spread the photoresist at a low speed of 800r / min for 6s; at a high speed of 3000r / min for 28s. Spreading the photoresist ensures that the photoresist is evenly coated on the GaAs surface. Centrifugal force is mainly used to rotate the photoresist from low speed to high speed and evenly coat the GaAs surface.

[0074] ② Pre-baking: solidify the photoresist and increase the adhesion between the photoresist and the substrate. This experiment uses 100℃ for 15min.

[0075] ③ Back side glue: for protection, to prevent subsequent process etching from reacting with the substrate, glue dripping, low speed, 800r / min, 6s; high speed, 3000r / min, 28s;

[0076] ④Pre-baking: 100℃, 15min;

[0077] ⑤ Exposure: After cooling, place the 3mm thick GaAs substrate on the photolithography machine, straight up and down, use a mask with a special pattern (small round bag shape), and expose for 30s to achieve pattern transfer;

[0078] ⑥ Development and rinsing: Place the GaAs substrate in a positive photoresist developer and develop for 1-2 minutes to obtain a pattern identical to that of the mask, then rinse in deionized water for 2-3 minutes;

[0079] ⑦ Hard film: 120℃, 25min.

[0080] 4. Etching

[0081] like Figure 3 As shown, this step utilizes etching or stripping liquid to achieve the purpose of pattern transfer, which is a very important operation for the electrical performance of the photoconductive switch.

[0082] The specific operations are as follows:

[0083] ① Use diluted aqua regia (HCl:HNO3:H2O=6:2:1) for etching. At room temperature, the etching reaction is complete, the GaAs substrate should be gray, the pattern in the exposed area is completely etched away, and the unexposed pattern is still protected by the photoresist. When the GaAs substrate appears light gray, take it out and rinse it in deionized water for 3-4 times. The purpose of this step is to etch away excess metal and obtain the required photoconductive switch electrode pattern;

[0084] ② Degumming: Remove the photoresist in the unexposed area of ​​the mask and the photoresist on the back of the substrate. Soak in acetone-ethanol-deionized water for 10 minutes each to degumming, and finally blow dry with N2.

[0085] 5. Annealing

[0086] Different metal materials have different properties. For the same metal, its internal properties can be changed through some processing methods. Annealing is one of the common heat treatment processes and is also a commonly used technology in semiconductor technology. After the preparation of the metal electrode on the surface of the semiconductor is completed, it is generally subjected to annealing treatment at the end because annealing can repair lattice defects and improve the metal surface morphology. In the early processes such as vacuum coating and etching, due to factors such as temperature, the arrangement method is different and the crystal surface is arranged irregularly. After annealing, its stress can be reduced to prevent deformation. At the same time, it has a certain effect on internal crystal arrangement, grain refinement, and reduction of structural defects. In addition, after annealing, the device can form a better ohmic contact, thereby improving the performance of the device. According to the literature, the annealing temperature of gallium arsenide is generally 380-460°C, but in actual operation, it is found that when the annealing temperature reaches above 400°C, gallium arsenide will sublimate, and black substances are found on the quartz tube wall of the annealing furnace. Such as Figure 4-5 As shown in the figure, two different GaAs photoconductive switches were annealed respectively. This experiment was to explore the best annealing temperature and time after Au was plated on the surface of GaAs photoconductive switch. The annealing temperature range was set at 230-590℃, the time was 1-7min, and the equal gradient exploration made the experiment more reliable and reasonable.

[0087] The specific experimental operation process is as follows:

[0088] ① Turn on the main power of the RPT annealing furnace, and the power button of the machine will light up;

[0089] ②Open the V1 valve, note that the vacuum gauge pointer should be at zero, place the annealed GaAs sample on the sample table in the quartz tube of the annealing furnace, close the annealing furnace, tighten the screws, tighten the three screws in sequence, and try to keep the same angle each time;

[0090] ③ Close the V1 valve, open the V2 valve, turn on the rotary vane vacuum pump, start pumping, keep the quartz tube in a vacuum state, and time the pumping for 5 minutes;

[0091] ④After the vacuuming is finished, start the purge. Open the N2 bottle, turn the main valve counterclockwise and the secondary valve clockwise to open, close V2, open V3, let the vacuum gauge reach zero, close V3, open V2, and pump to near the 0.1 mark, repeat this step 5 times, and finally keep V2 open;

[0092] ⑤ Open V3 and adjust the gas flow to keep it at scale line 4. The vacuum gauge pointer should be between the second and third scale lines.

[0093] ⑥ Turn on the lock button, adjust the annealing temperature and time, set the program, turn on lock on, press and hold run, annealing starts, annealing temperature settings: 230℃, 350℃, 470℃, 550℃, annealing time settings: 1min, 3min, 5min, 7min, a total of 16 groups of experiments, 16 samples;

[0094] ⑦ After annealing, close the N2 bottle, then close V3 and V2, turn off the rotary vane vacuum pump, open V1, deflate, loosen the screws, and take out the sample;

[0095] ⑧ Turn off V1, turn on V2 and the vacuum pump, pump for 5 minutes, turn off V2 and the vacuum pump, press turn off, then press lock to turn off the power switch.

[0096] Surface electrodes Ge, Ni and Au

[0097] Select 4 inches, crystal orientation <100> GaAs substrate, substrate thickness 2mm, 3mm, resistivity ≥107Ω·cm, use a chip dicing machine to cut a wafer of the required size as a substrate, the chip substrate size is 10mm×20mm; use acetone, ethanol, and deionized water to ultrasonically clean the surface of the GaAs substrate in sequence; place a stainless steel metal mask frame with a pre-designed electrode pattern flat and close to the surface of the GaAs; use vacuum coating electron beam evaporation technology to prepare a switch electrode on the surface of the substrate, and the material of the electrode is made of high-purity metal particles. The sample is taken out of the vacuum chamber and a stainless steel metal mask frame is placed on the other surface opposite to the gallium arsenide. The electron beam coating technology is continued to be used to make another metal electrode. The thickness of the plated metal film corresponds to the thickness of various films of the previous electrode. The switch chip with metal electrodes on both sides is placed in a rapid annealing furnace for stress relief annealing to make an alloy. The annealing temperature is 460°C and the holding time is 120s.

[0098] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a gallium arsenide semiconductor photoconductive switch electrode, characterized in that: The following steps are involved: (1) Using a dicing machine to cut a wafer of the required size as a substrate; (2) Cleaning the surface of gallium arsenide; (3) A stainless steel metal mask frame is attached to a surface of the gallium arsenide; the stainless steel metal mask frame is processed into a pattern required for the switch electrode by semiconductor photolithography and etching methods; (4) The coating is used as an electrode; the coating technology adopts vacuum electron beam evaporation coating technology or magnetron sputtering technology to sequentially coat Ge, Ni and Au films; the coating technology adopts vacuum electron beam evaporation coating technology or magnetron sputtering technology; (5) A layer of stainless steel metal mask frame is closely attached to the other surface opposite to the gallium arsenide; the stainless steel metal mask frame is processed into the pattern required for the switch electrode by semiconductor photolithography and etching methods; (6) coating as another electrode; the coating technology adopts vacuum electron beam evaporation coating technology or magnetron sputtering technology, and Ge, Ni and Au films are coated in sequence; the coating technology adopts vacuum electron beam evaporation coating technology or magnetron sputtering technology; (7) The switch chip with metal electrodes on both sides is placed in a rapid annealing furnace for stress relief annealing and alloy treatment.

2. The method for preparing a gallium arsenide semiconductor photoconductive switch electrode according to claim 1, characterized in that: In the steps (3) and (5), the edges of the electrode pattern in the stainless steel metal mask frame are smooth, free of burrs and sharp right angles.

3. The method for preparing a gallium arsenide semiconductor photoconductive switch electrode according to claim 1, characterized in that: The thicknesses of the Ge film, the Ni film and the Au film are 5-200nm, 50-4000nm and 200-2000nm respectively.

4. The method for preparing a gallium arsenide semiconductor photoconductive switch electrode according to claim 1, characterized in that: In the steps (4) and (6), an Au film is plated, and the thickness of the Au film is 200-2000nm.

5. The method for preparing a gallium arsenide semiconductor photoconductive switch electrode according to claim 1, characterized in that: In the step (7), the stress relief annealing treatment is performed in a rapid annealing furnace, and the temperature of the stress relief annealing is 250-480°C.

6. The method for preparing a gallium arsenide semiconductor photoconductive switch electrode according to claim 5, characterized in that: The insulation time is 30s to 900s.

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